1. Field of the Invention
This invention relates to a small metal vapor discharge lamp, and more particularly to a metal vapor discharge lamp of a single end type such as a small metal halide lamp.
2. Description of the Related Art
The conventional metal vapor discharge lamp of a general type such as a metal halide lamp produces a large output and has a large arc tube which is of a double end type.
Recently, a metal vapor discharge lamp such as a small metal vapor discharge lamp such as a metal halide lamp producing a small output. The developed lamp has a small arc tube which is of a single end type.
With the small lamp, the ratio of the electric power supplied to the lamp to the surface area of the discharge space, that is the lamp load, is set to such a high value as 20 to 70 W/cm.sup.2 so as to increase the efficiency of the lamp. In order to miniaturize the arc tube, the distance between the electrodes is rendered much smaller than that of the lamp which has a conventional arc tube of a double end type. Accordingly, much metal such as mercury which is sealed in the discharge space is required to obtain necessary lamp characteristics such as the predetermined lamp voltage and the lamp power, inevitably rendering the pressure in the arc tube at the time of lightning to such a very high value as 15 kg/cm.sup.2.
In consequence, the arc tube of a small metal vapor lamp must withstand a high temperature and a high inner pressure. The arc tube of a general type is made of quartz glass. When it is disposed under a severe condition as described above at the time of lightening, it is deformed and changes its inner volume, leading to the change of the lamp characteristics and the breakage of the lamp. Further, when the small arc tube is subjected to a high lamp load, the temperature of the coolest portion is apt to be uneven and the lamp characteristics are likely to be varied.